Hip joint endoprosthesis

ABSTRACT

An endoprosthesis for a hip joint includes an acetabular implant and a femoral implant. The acetabular implant has a socket with a socket opening intersecting the rim of the socket in a plane. At least two tubular guides for positioning at the cranial region of the bone are attached to and laterally offset from the socket opening, have axes perpendicular to the socket plane, and slidingly receive attachment screws the ends of which are threaded into the bone. The femoral implant has a spherical head movably positioned in the acetabular socket which is mounted to an attachment member the distal side of which has a conical attachment surface bearing against the resected femoral neck. A sliding rod extends coaxially with the femoral neck axis laterally and distally into a sleeve connected to a plate which is secured to the femur.

BACKGROUND OF THE INVENTION

The invention relates to an endoprosthesis, in particular for a hipjoint, having an anchoring component which can be anchored to the boneand which is connected to a joint component. Endprostheses are implantedwhen a satisfactory therapeutic effect can no longer be achieved byoperations performed to preserve the joint. This is particularly thecase where hip joints are concerned when advanced arthropathy, necrosisof the head of the femur or a medial fracture of the neck of the femurare present.

One disadvantage of all known endoprostheses is that, although these areinitially immovably joined with the bone, they can become loose after acertain amount of time. Living bone is constantly undergoing changeswhich may vary considerably from location to location. If, in the courseof such changes, individual sections of the bone should become weakenedin the peripheral area of the prosthesis, the bone at this point willgive way and the prosthesis will move. Even if such movements occur onlyin the micro-range, the original stability is no longer guaranteed.Under unstable conditions, the force flows and load distributions on thebone are constantly changing and the bone cannot start any reparativereactions because the conditions for oriented growth are lacking.Instead, at the points where excessive stress builds up (stressreaction) the bone will react by locally atrophying as it does at pointswhere the stress is too low (stress protection). This leads to aprogressive loosening of the prosthesis.

Such restructuring of the bone must be expected to occur even yearsafter the prosthesis has successfully healed into place, because bone isa living organ and it reacts to changes in lifestyle, diet and othercircumstances. Conventional endoprostheses do not take account of thissituation.

A loosening process of this kind is one of the major problemsencountered in orthopaedic surgery and it frequently necessitatesreplacement of the prosthesis. The corrective operations required inthis case are, however, much more problem-ridden than the primaryinterventions because not only is the bone tissue which was removed forthe primary implantation now missing, but also usually as a result ofthe interventions because not only is the bone tissue which was removedfor the primary implantation now missing, but also usually as a resultof the loosened implant or because of the resulting abrasion, additionalserious defects are created which make it extremely difficult or indeedimpossible to re-fix the implant.

In the case of hip endoprostheses sometimes only the head and neck ofthe femur are removed and replaced by a head prosthesis.

In the prior art head prostheses the anchoring component consistsusually of a metal spike, for example of titanium, which is fixed in themedullary cavity of the femur and it is fitted with a laterallyprojecting pin whose free end bears a head section. In order to implantsuch a prosthesis it is necessary to remove the head and neck of thefemur as well as most of the sponglous bone at the proximal end of thefemur. The spike is then cemented into the femoral medullary cavityusing polymethylmethacrylate, or it is driven into the bone without anycement; in this latter case it is primarily necessary to achieve optimalcontact between the bone and the implant, and this may be reinforced bysecondary growth of new bone tissue.

German Patent Application DE-A 28 45 231 describes a joint prosthesis inwhich a joint component is provided with a shaft which is attached tothe bone by means of a tension bolt.

German Patent Application DE-A 28 54 334 describes an endoprosthesis ofcomplicated design for a hip joint. The prosthesis consists of a shaftrunning longitudinally in the medullary cavity of the bone, and a neckpart running in the direction of the neck of the femur and carrying thehead of the joint. In this design, the shaft in the medullary cavity isrigidly connected to the femoral neck part.

German Patent Application DE-A 30 17 953 reveals an endoprosthesis forthe head of a femur in which the head component is rigidly andnon-slidingly connected via a threaded bolt with an anchoring componentconsisting of a sleeve inserted into the bone and of a trochanteralplate bolted to the outer surface of the bone. In this case the threadedbolt acts as a tension anchor by means of which the prosthesis ispretensioned against the bone.

German Patent Application DE-A 34 20 035 describes a joint prosthesis inwhich the head of the joint is rigidly attached to the bone via aprojecting member.

One common feature of all these prior art joint prostheses is that theyare non-flexibly attached to the bone in a manner which does not allowfor later restructuring processes taking place in the bone.

When an acetabular prosthesis is implanted, still intact bone materialis sometimes removed from the pelvic bone in order to be able to cementthe socket into place or to anchor it in position without the use ofcement. This known method of carrying out the operation, using knowntypes of endoprosthesis, has a number of disadvantages. For example,large amounts of intact bone material have to be sacrificed, includingin particular those parts of the bone in the proximal area of the femurwhich, because of their ideal trajectory, provide for optimum absorptionof forces.

In the area of the femoral shaft, the existing force flows arereoriented. Under natural conditions, mainly pure bending stresses,harmoniously distributed from top to bottom, predominate in this area,but when the prosthesis has been implanted forces are generated whichrun mainly from inside the bone to the outside and these are combinedwith shear forces and a relatively abrupt transition from low stress inthe portions of the bone in contact with the prosthesis to extremestress at the lower tip of the anchoring spike. The bone itself isforced to react to the changed conditions by undergoing restructuring.This applies not only to the femoral but also to the acetabularconditions. Restructuring always involves the simultaneous loss ofexisting bone and the growth of new bone. If the amount of bone lostexceeds the amount of new bone growth, the prosthesis will lose its gripin certain sections and it will start to come loose.

Also, when the prior art hip joint prostheses are used, insufficientattention is paid to the individually different conditions in thepatient's anatomy. For example, the angle between the shaft and the neckof the femur ("CCD angle") varies in size from person to person (thephysiological range is taken to be 115° to 140° degrees). Furthermore,the neck of the femur does not run in the sagittal plane of the body butis tilted forward and towards this plane at an angle of varying size("AT angle", physiological range approx. 10° to 40° degrees). The sizesof both angles are proportionately interdependent. Prior artendoprostheses do not take these facts into account and only onestandard dimension is used for all hips. As a result, this almost alwaysleads to functionally incorrect placing of the joint and consequentlyalso a change in the way in which forces are introduced into the shaftof the femur. To the extent that they are capable, bones and softtissues must adapt to the new conditions. Pain, restricted mobility andpremature loosening of the prosthesis are all possible consequences.Custom-made prostheses are not immediately available, they are for themost part inaccurate, and they cost up to ten times as much.Furthermore, computer tomography, which exposes the patient to largeamounts of radiation, is needed to determine the anatomical situationprior to performing the operation. In this case, too, the functionallyincorrect siting of the joint involves the risk that the implantedprosthesis will come loose, with all the disadvantages already mentionedabove.

Colonization of the surface of the prosthesis by bacteria is a notinfrequent and much feared complication following implantation. Suchcontamination usually makes it necessary to remove the prosthesiscompletely. However, removing an endoprosthesis which has become firmlyinterlocked with the bone growing around it, or removing the bonecement, is a very difficult task which frequently results in severeadditional damage to the surrounding bone structures. Replacing theprosthesis with a new one cannot be done at all or only after the bonehas been allowed to heal for several years. If an infection exists inthe area of a femoral head prosthesis of prior art design, then becausethe prosthesis is anchored in the medullary cavity of the femur it mustbe expected that the infection will spread as far as the knee joint.

Anchoring the head component of the prosthesis in the shaft of the boneis not only a non-physiological approach but also it is not alwaystechnically simple to accomplish. Usually, an extensive set ofadditional surgical instruments is needed to prepare the bone in theexact manner required. Furthermore, opening up the medullary cavityalways results in heavy and persistent bleeding which necessitates theadministration of multiple units of stored blood.

SUMMARY OF THE INVENTION

It is a purpose of the present invention to avoid the aforementioneddisadvantages and to create an endoprosthesis which is able to adaptitself in a controlled manner, in particular to any changes which mayoccur at a later date in the bone. To solve this task in the manneraccording to the invention it is proposed that, proceeding from anendoprosthesis of the type described at the beginning, either theanchoring component or the joint component should possess a tubularguide, and that the other one of these two components should possess arod-shaped sliding component which is slidingly mounted in the tubularguide, in the path of the physiological force flow. The design of theendoprosthesis according to the invention thus guarantees the guidedsliding motion of the joint component relative to the anchoringcomponent, namely in an exactly predetermined axial direction, i.e. thedirection of flow of the physiological forces, throughout the entireperiod of time that the endoprosthesis is in use. This offers two mainadvantages. On the one hand, the guided sliding motion permits axialdisplacement in the event that localized bone loss occurs, so that theprosthesis remains stable over a practically unlimited period of timefollowing implantation; in contrast, when the prior art prostheses areused, there is a risk that, because of restructuring processes takingplace in the living bone after the prosthesis has healed into place, thesaid prosthesis will become loosened or no longer optimally positioned;on the other hand, the arrangement according to the invention offers thefurther advantage that the prosthesis automatically adjusts itself tothe optimum position, thereby ensuring an optimum distribution of theload between the prosthesis and the bone. In the design according to theinvention, the anchoring component plays only a subordinate role inabsorbing the force flows. Instead, as in the natural joint, the forcesflow through the preformed bone structures.

The head prosthesis according to the invention, which consists of ananchoring component attached to the femur and joined via a neckcomponent to a head component attached thereto, is characterized in thatthe neck component is connected to the rod-shaped sliding componentwhich is slidingly arranged, substantially along the axis of the femoralneck, in a tubular guide connected to the anchoring component, and thesliding component is provided with an attachment bearing on the neck ofthe femur. This ensures that a physiologically correct force flow iscreated, so that even after the prosthesis has been implanted the forcesare naturally distributed in the femur and no individual section of thebone is subjected to non-physiological load stresses to which it reactsby undergoing restructuring.

At the same time, the attachment element provides reliable anchoring ofthe prosthesis to the femur without it being necessary to remove largeamounts of intact bone material.

In accordance with a preferred embodiment of the invention, the surfaceof the attachment bearing on the neck of the femur has the form of atruncated cone, the vertex of the cone has a lateral-distal orientation,and the axis of the cone coincides with the axis of the rod-shapedsliding component. The vertex angle of the cone is advantageouslybetween 135° and 140°. This ensures that the surfaces of the attachmentin contact with the bone are everywhere substantially perpendicular tothe preformed trabecular structures. This also guarantees that theanatomically predetermined ratios between the CCD and AT angles areretained after the prosthesis has been fitted. The forces are thereforetransmitted in a substantially physiologically correct manner over thenaturally existing structures.

In accordance with a further feature of the invention, the neckcomponent possesses a cone element arranged centrally to the truncatedconical attachment; the axis of this cone element coincidessubstantially with the axis of the femoral neck and the cone is insertedinto a conical recess in the head component of the prosthesis. By meansof this design it is possible to use head components from prior artprostheses in the endoprosthesis according to the invention, i.e. thislatter endoprosthesis can be combined with prior art types of headcomponents.

In accordance with a preferred embodiment of the invention, theanchoring component possesses a plate of known design, for use withdynamic hip screws, which is attached laterally to the femur. This plateis connected to the tubular guide which passes through the neck of thefemur in a substantially axial direction and in which the slidingcomponent is slidingly located. With this arrangement, it is notnecessary to remove large portions of still intact bone material, thusthe amounts of bone sacrificed in the previously known types ofendoprosthesis are for the most part saved. All that is necessary is toarrange a borehole in the axis of the femoral neck, through which thesleeve can be inserted. The anchoring component is fixed in position bylaterally screwing the plate to the bone, thus obviating the need forlaborious preparation of the femur using a complex set of instruments.Because the anchoring component is not anchored in the medullary cavityof the femur, persistent bleeding from that cavity is avoided and theneed for blood transfusions is considerably reduced, indeed, they maynot be necessary at all. If bacteria infect the area, the anchoringcomponent can easily be removed and it is not possible for an infectionprocess to spread over the entire femur, consequently immediatereimplantation of a new prosthesis is a much less risky procedure. Thisdesign offers the further advantage that if the endoprosthesis accordingto the invention should ever fail, it can be replaced without anyproblem by a prior art type of prosthesis as if the operation were beingperformed for the first time, because the implantation according to theinvention does not create any defects in the needed bone substance.

The acetabular prosthesis according to the invention is based on a knowntype of socket anchored in a recess cut out of the pelvic bone andhaving at its cranial margin at least one bracket which can be attachedto the pelvic bone by fastening devices. The said acetabular prosthesisis characterized in that at least two tubular guides connected with thebracket or brackets are provided, with their axes running perpendicularto the plane of the socket opening, and through these guides pass screwshaving rod-shaped sections slidingly arranged relative to the tubularguides, which can be anchored in the pelvic bone. This design alsoensures that, if the bone structure undergoes any changes, the slidingmounting can permit axial displacement of the socket without allowingthe latter to tilt or rotate, so that even in cases of localized boneloss, the prosthesis remains stable at all times, without the flow offorces between the prothesis and the bone being disrupted orinterrupted. Even if the prosthesis should undergo any settling, all thecircumferential parts of the socket continue to be uniformly loaded. Incontrast, in the known types of prosthesis there is a risk of tiltingoccurring, which would interfere with the flow of forces. Furthermore,once the screws have been inserted, this design permits the socket to bedisplaced perpendicularly to the plane of the socket opening, however atthe same time it prevents the socket from tilting or rotating.

In order to ensure that the thread of the screws does not prevent thesliding motion in the axis of the screws, the latter--in accordance witha further feature of the invention--possess a central threaded anchoringsection and a peripheral sliding section slidingly mounted in theassociated sleeve.

When preparing the bed in the bone for the acetabular prosthesisaccording to the invention, it is no longer necessary as in the past toremove all the sclerotic portions of bone or even to create additionaldefects such as a conical or threaded preparation of the bone, insteadit is sufficient to prepare a congruent hemispherical cavity usingconventional rasps and to remove any remaining fragments of cartilage,because inhomogeneities in the bone structure can be balanced out againthanks to the self-regulating characteristic of the acetabularprosthesis according to the invention. In this case, also, the sacrificeof still intact bone material is reduced to an absolute minimum.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a complete endoprosthesis for a hip joint; and

FIG. 2 shows a view of the acetabular prosthesis seen in the directionof the arrow II in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The complete hip endoprosthesis illustrated here possesses a headprosthesis 1 and an acetabular prosthesis 2.

The head prosthesis 1 consists of a head component 3 mounted on a cone 4which is connected to an attachment 5 having a truncated conical surface6 which rests in a recess cut into the neck of the femur 7. The vertexof the truncated cone is thus laterally and distally oriented and theaxis of the truncated cone coincides substantially with the axis 8 ofthe femoral neck 7.

The vertex angle a of the truncated cone is between approximately 135°and 140°, so that the surface 6 of the attachment 5 lies approximatelyperpendicular to the preformed trabecular structures.

The cone 4 and the attachment 5 together form a neck component which isconnected to a rod-shaped sliding part 9 emerging from the vertex of thetruncated cone and slidingly guided along the axis of the femoral neck 8in an anchoring component 10. For this purpose, the anchoring component10 possesses a sleeve 11 which is inserted into a laterally-distallyopen borehole in the neck of the femur 7 and oriented along the axis ofthe neck of the femur. The sleeve 11 is connected to a plate 12 providedwith holes through which screws 14 are passed and then screwed into thefemur 13.

The acetabular prosthesis 2 consists of a hemispherical socket 15 madeof a highly biocompatible metal such as titanium, and it is providedwith customary fastening devices 16 for securely inserting an inlay 17made of slippery material, for example polyethylene. As can be seen fromFIG. 2, the socket 15 is connected at its cranial margin to two brackets18, each of which is fitted with a sliding sleeve 19, and the axes ofthese sliding sleeves run perpendicular to the plane of the socketopening 20. However, each bracket 18 may also possess two or moresliding sleeves 19 through which pass screws 21, and it is possible alsoto provide just one bracket with at least two sliding sleeves 19. Afterthe socket 15 has been implanted, it is fixed in place in the acetabulumby means of the screws 21 passing through the sliding sleeves 19. Thescrews 21 possess a central threaded anchoring section 22 and aperipheral sliding section 23 which is slidingly located in the sleeves19.

This arrangement guarantees that the socket 15 is fixed so as to preventit from tilting and rotating but, because the axes of the screws areperpendicular to the plane of the socket opening 20, it also permitsaxial displacement in the event of localized loss of bone and thus thestability of the prosthesis is maintained.

The following procedure is followed when implanting the completeendoprosthesis as shown in the drawing:

Once the joint capsule has been opened up and following luxation of thehead section of the hip, the latter is resected at the cartilage-boneinterface, ensuring that the resection plane remains as closely aspossible perpendicular to the axis of the femoral head 8. In the case ofa medial fracture of the neck of the femur, the broken head is firstremoved and then only the larger cortical splinters are smoothed down inthe area of the neck stump. Next, starting from the resection surface, adrill guide wire is centrally positioned along the axis of the neck ofthe femur 8. The wire passes through the entire neck section 7 andemerges laterally from the femur 13. The central position of the wirecan be ensured by using an aiming device or by carrying out X-raymonitoring (image converter). Subsequently, drilling is carried outusing a multi-cut drill bit, starting laterally and moving along thelength of the wire. In this way a borehole of large diameter is producedfor the sleeve 11 of the anchoring component 10 and a borehole ofsmaller diameter is produced to accept the sliding component 9. Theangle between the borehole and the shaft of the femur is measured andthe anchoring component 10 is selected in accordance with this angle; itis then inserted and attached by screws 14 to the femur 13. Next, thebearing surface for the attachment 5 is prepared from the medial endusing a rasp of truncated conical shape corresponding exactly to theshape of the attachment 5 and having at its vertex a guide rod which isintroduced into the already inserted sleeve 11. The preparation processcontinues until all the cortical edges of the neck stump of the femurhave been smoothed down.

The bed for the joint socket is prepared using a conventionalhemispherical rasp whose diameter should correspond to the maximumdiameter of the acetabulum. A test socket of identical shape is fittedinto the bed thus formed; this socket is provided at its cranial marginwith at least two guide bushings for a drill running perpendicular tothe plane of the socket mouth. Once the optimum position of theacetabular prosthesis to be implanted has been determined, for exampleby tilting or rotating the test prosthesis, the boreholes for acceptingthe screws 21 are drilled through the guide bushings. The testprosthesis is then removed and the length of the boreholes is measured.Using a countersink drill having a constant drilling depth, theperipheral sections of the boreholes are widened to accept the slidingsleeves 19 of the acetabular prosthesis 2. Then the actual acetabularprosthesis 2 of appropriate diameter is inserted in such a way that itssliding sleeves 19 come to rest in the recesses prepared for them. Theprosthesis is firmly hammered into position and then fixed in place bymeans of the screws 21 of suitable length passing through the slidingsleeves 19. The sliding section 23 of the screws 21 must always projectbeyond the central end of the sliding sleeve 19.

The head prosthesis 1, which has in the meantime been got ready, is theninserted from the medial end, with the sliding component 9 beingintroduced first, into the bed which has been prepared for it in thebone and it is then hammered firmly into place. The peripheral end ofthe sliding component 9 should then end near the lateral opening of thesleeve 11. Once the head component 3 has been hammered onto the cone 4,the head component 3 is placed in the acetabular prosthesis 2 and theimplantation process is complete.

Although the drawing depicts a hip endoprosthesis, the invention can, inprinciple, be applied with the same advantages to prostheses for otherjoints, in particular the knee joint. In all types of prosthesis theessential feature is that a joint part is slidingly guided in the axialdirection in relation to an anchoring part, so that in the event oflocalized bone loss occurring, the parts of the prosthesis are preventedfrom tilting or twisting, without any interruption in the flow of forcesthrough naturally existing bone structures.

In the area of the knee joint the anchoring component takes the form ofone sliding sleeve implanted in the femoral shaft and another in thetibial shaft. A preferably rod-shaped sliding component securelyconnected to the femoral or tibial joint component, which may be of anysuitable design, is slidingly located in this anchoring component.

In the shoulder area, the humeral anchoring component is fastened, in amanner similar to that used for the hip joint, to the lateral surface ofthe humerus. Again, a preferably rod-shaped sliding component, which isrigidly connected with a hemispherical joint component, is slidinglyguided in this anchoring component. The glenoidal part of the prosthesisconsists advantageously of a sleeve-shaped anchoring component which isfixed in the shoulder blade and in which a preferably rod-shaped slidingcomponent, rigidly connected to a dish-shaped joint component, isslidingly guided.

I claim:
 1. A hip joint prothesis comprising a hemispherical socket configured to be anchored in a recess of an acetabulum, the socket having a socket opening terminating at a peripheral rim and defining a plane at the rim, at least two elongated tubular guides each to be located at a cranial margin of the bone and having a longitudinal axis, means connecting the tubular guides to the socket and laterally of the socket opening so that their longitudinal axes are perpendicular to the plane of the socket opening, and fastening means having a proximal portion slidably disposed in the tubular guides and a distal portion for anchoring the socket to the bone.
 2. A hip joint prosthesis according to claim 1 including a femoral head component for being slidably disposed in the socket, a neck member connected to the head component including a conical attachment surface having a vertex and facing away from the head for positioning against resected bone of the femur in substantial alignment with an axis of the femoral neck, an anchoring member for attachment to the femur and positioned laterally and distally of the femoral neck and including a tubular guide extending into the femur in alignment with the femoral neck axis, and an elongated guide member secured to the anchoring member, projecting coaxially with the femoral neck axis from the attachment surface and being slidably disposed in the tubular guide.
 3. A hip joint prosthesis according to claim 2 wherein the conical attachment surface has a vertex angle of between 135° and 140°.
 4. A hip joint prosthesis according to claim 2 wherein the anchoring member includes a tapered shaft projecting in a direction opposite to and coaxially with the guide member, and wherein the femoral head component includes a tapered bore adapted to receive the tapered shaft, the taper of the shaft and the bore being selected to secure the head component to the shaft. 